This publication brings to the readers practical answers
to welding problems in an informal setting designed to be
helpful and informative. We actively seek feedback to make
it ever more useful and up to date. We encourage you to
comment and to contribute your experience, if you think
it may be useful to your fellow readers.

You are urged to pass-along this publication to your
friends, if you like it, and if you want to help them.
If you received this from a friend and if you like
what you read, please subscribe free of charge and you
will also receive a bonus book on
Practical HARDNESS TESTING Made Simple.Click here.

This Issue of Practical Welding Letter is now addressed to
an audience of almost 500 Readers: it is a motive of pride
to see subscriptions grow steadily from week to week, with
no advertisement other that the invitations appearing in the
Welding Advisers Site and our Readers spreading the word.

Although we have no way of monitoring the diffusion efforts
done by our Readers towards their friends and colleagues,
we invite you to forward this very page to anybody you think
might be interested in the information included. Thank you.

We are always looking for new ways of providing useful and
timely knowledge. We are helped, if you wish to collaborate,
by your feedback but also by your questions, that cover a
large and increasing range of subjects.
We can assure you that we will study every feedback/request
and that we will do our best to satisfy your curiosity and needs.

In the present issue of Practical Welding Letter we introduce
an Article on Inverters, to explain their most
basic principles of operation and the rewarding fields of their
use, while warning, as usual, against the incorrect perception
that they might be the solution to any problem.

We show a possibly overlooked aspect of fillet welding, indicating
that it may be used to one's advantage with a certain class of steels.

Selecting Filler Metals for Hardfacing may be difficult
and confusing because every application has its own
requirements and constraints. The information provided
should help in asking the right questions, the first
necessary step in trying to obtain significant answers.

Related to this subject are Terms and Definitions presented
in this issue to clarify the very conditions that Hardfacing
is called to withstand.

An Article on Receiving Inspection may provide some thoughts
about a Quality activity that may seem superfluous, except that,
if overlooked, it may be costly and painful when something
goes wrong.

The usual Sections, of course, are where you expect to
find them...

And for the Gift and Bonus we promised, when you finish
reading this newsletter, find them in the Bulletin Board,
which is Section 12 down towards the end of the publication...

Welding Power Supplies based on Inverter Technology were made possible by using
Power switching Semiconductor devices called IGBT (Insulated Gate Bipolar Transistors, that are are voltage-controlled power transistors).

The drive of the development was provided by the fundamental principles of transformer construction that permit to reduce the bulk of iron core and copper
windings in direct proportion to the frequency of the primary voltage: the higher the
frequency, the higher the possible reduction of the mass of the transformer for the
same output. Therefore while a welding transformer of a given power for input
frequency of 50 or 60 Hz is bulky and heavy, a welding transformer for input
frequency of 20 to 100 kHz (kilo Hertz), for the same power, is small and light.

In welding power supplies based on inverter technology, the input alternating voltage
and current from the grid (at 50 or 60 Hz) are rectified to direct voltage and
current.
The rectified input is then passed through a power inverter that, by switching the
circuit rapidly on and off, produces high frequency voltage and current of the order
of thousands of Hz, typically 20 to 100 kHz. The high frequency voltage is reduced by
a step down transformer to a value suitable for welding applications, while the
corresponding current available is proportionally increased. In DC power supplies the
current is again rectified and filtered to a smooth and constant output.

The main advantage of inverters is their portability, because of their much reduced
bulk and weight. Therefore they are ideal for frequently moved field work and for
repair and maintenance. Another advantage derives from their energy consumption which
is less than for regular transformers. The primary circuit of a transformer is always
connected, even when not welding, so that a part of the energy drawn while idling
goes into wasted non recoverable heat. In inverters energy is consumed only when
actually welding.

There may be some disagreement about the effective energy savings
that can be realized but they are certainly more important for continuous operations.
Older inverters supplied only DC, but later new functions and features were
incorporated in these power supplies, providing pulsed DC, jumping continually
between a lower level of current (background current), established to maintain the
heat, and a peak current level that actually melts the metal.

This feature permits a
level of heat control unheard of before, permitting heat input to be reduced when
needed, especially with aluminum (between 1 and 2.5 mm or 0.040" and 0.100") or for
thin gauges in all metals. Pulse width, that influences drop size and arc cone width,
and pulse frequency, that influences average amperage, heat input and arc length, can
also be controlled.

It is claimed that pulsed GMAW reduces spatter, increases weld deposition rate,
lowers fumes and improves appearance. The arc provided by an inverter based AC power
supply is so stable that it normally does not need superposed High Frequency except
for starting, so that there is no Radio Frequency Interference.

When AC features were finally introduced in inverter equipment, they permitted
freedom from the rigid frequency from the grid, allowing to play with frequencies
from 20 to 250 Hz. It resulted that the characteristics of the arc are changed so
that for every application the most suitable AC frequency may be selected. A higher
frequency produces a much stiffer arc that permits precise emplacement and deeper
penetration, while lower frequency allows a softer arc with best cleaning properties
producing a wide and smooth weld bead. This is also useful for depositing metal for
rebuilding or for hardfacing.

It was long known that the electrode positive portion of the AC cycle provides a
cleaning action, disrupting surface oxides, while the negative portion provides for
metal heating, fusion and penetration for a deeper, narrower bead. This was the case
of the balanced wave, where 50% of the time the electrode was positive, and 50% was
negative. But then it was found that also the time duration of each portion of a
cycle could be manipulated, so that reducing the percentage of time with electrode
positive well below 50% still provided acceptable cleaning action, while increasing
the percentage of time at electrode negative increased heat input and weld
deposition.

But besides this advantage, a secondary gain was also achieved.
In GTAW the Tungsten electrode is heated mostly during the electrode positive portion
of the cycle. The introduction of the unbalanced cycle, reducing the time of
electrode positive, permitted less heating of the same, allowing either the use of a
smaller size electrode or maintaining its pointed, sharpened shape, without growing a
drop at its end which promotes arc instability and interferes with proper welding
practice.

One more feature proposed by manufacturers, but only interesting when the power
supply is frequently moved around, is the capability of certain modern inverter based
machines to accept any one of a wide range of voltage inputs, with automatic
adjustment freeing the welder from the concern of adapting the connections to the
actual conditions.

A basic welding power supply based on inverter technology may provide DC welding
current up to about 200 amp for essential applications of SMAW and GTAW for steel.
It is a unit of very light weight, that can be easily moved around and brought to
the large pieces needing construction or repair.

More elaborate SMAW-GTAW units, with AC/DC capability, usually with higher current
output up to 300 amp, may provide pulsing of the direct current between a subsistence
lower level and a peak current level: this feature may be handy for GTA Welding of
thin sheets and tubing with low heat input.
The maximum flexibility available is provided by an inverter based power supply that
is designed to allow its usage for all manual processes, not only SMAW and GTAW, but,
with a suitable wire feeder, also GMAW and FCAW. This universal power supply, offered
for up to 400 amps, makes sense for any welder wanting to be able to select the most
suitable process for the job at short notice, in any given situation: mostly for
maintenance and repair work in a large facility with many different requirements.

Obvious any application requires the process specific accessories.

If it is determined that inverter technology is needed for one or more of the
advantages listed above, then the suitable unit has to be selected taking into
account the processes the unit will serve, and the maximum thickness that one needs
to weld in one pass at the appropriate duty cycle (the actual percentage of arc on
time, in any given ten minutes period).

Summing up, the advantages offered by the implementation of inverter technology, must
be evaluated against the higher cost not only of the unit, but also of its expensive
repair that is a fact of life.

A: Rimmed Steel manufacturing processes provide a case or rim of very clean material
free of defects. Conversely impurities tend to concentrate in the middle section of
ingot or billet. This feature persists through the rolling process, so that plates of
this kind tend to have their central core less clean than the superficial layers.
This property provides and advantage when design calls for fillet welding, which does
not penetrate to the center of the plate.

Note - Hardfacing Material are used for providing working
surfaces of implements or machines with improved properties
making them suitable to resist the destructive actions of
forces acting on them. These harmful actions are best
described by Terms and Definitions grouped for your
convenience in section 6 hereafter in this publication.
------------

A general introduction to the subject of Hardfacing is presented
in the Welding Advisers Site: Click here.

Here we intend to provide some more information that should
be helpful to make an informed selection of the types of filler
materials appropriate to a given situation.
After having determined the general classes of materials
most probably suitable to provide an acceptable solution,
the inquirer should contact a few of the best known
manufacturers and ask them to offer their products for a
definite application.

Then, with the information obtained, the total expense for
a given job should be estimated for every one of the products,
as indicated in the above page of the site, and again at the
end of this article. The calculations permit to arrange the
products in a list in the order of increasing total cost.

Finally, based on previous experience and comparison with other
known cases the final selection should be decided, trying
to opt for the minimum total cost that ensures the longest
and most satisfactory performance.

It should be appreciated that more than one acceptable solution
may be applicable to any given situation and that the actual
operating conditions are the essential variables that govern
the selection of the most suitable hardfacing products and
processes. Different welding processes are suitable for
hardfacing. The selection is usually based on availability,
on the dilution obtained (which should be kept to a minimum)
and on the deposition rate.

AWS A5.13 and A5.21 provide classifications for limited
classes of Filler materials. Most of the available alloys
are provided by manufacurers under trade names.
It is therefore important to investigate with the suppliers
which materials they would help you select for your specific
applications. And then nothing can take the determinant
place of comprehensive comparative tests.

The structure of the deposited metal usually consists in a
basic metallic soft matrix providing support to a hard phase
in the form of hard carbides, borides or intermetallics
designed to resist abrasive wear and other surface damage.
Matrix materials include low alloy steels, high alloy iron
base alloys, white irons, cobalt or nickel alloys and,
less commonly, copper alloys.

Iron is the least expensive matrix material, and it can be
found in a great number of proprietary alloys. Due to the
large variety of iron base alloys available for hardfacing
applications, it has become customary to group them more
by behavior under wear than by chemical composition.

Pearlitic steels are low-alloy steels. They contain low carbon
(<0.2% C) and low amounts of other alloying elements (up to 2% Cr),
and are useful as buildup overlays, to rebuild parts back to size.
This group of alloys has high impact resistance and low or medium
hardness (in the range of 25 to 37 HRC), as well as excellent
weldability. One typical alloy is designated E-Fe1.
They are not designed to resist metal wear but to provide support
for real hardfacing material.

For building up Austenitic Manganese steel, which is highly
resistant to impact and work hardens during usage, two types
of filler manganese steel are used, containing also Nickel
and Molybdenum: while Manganese is around 15% for both,
Chromium may be on the low side (about 4 %)(EFeMn-C)
for build up of machinery parts subjected to impact,
or on the high side (about 15%)(EFeMn-Cr), used for buildup
or welding to the same or other metals.
In any case welding has to be performed with the least
possible heat, while cooling by appropriate means the
surrounding structures.

For metal to metal wear resistance applications, martensitic
steels similar to tool steels are employed, with due
precautions during welding to avoid cracking: these, called
also machinery hardfacing alloys, harden upon cooling from
welding temperature and exhibit higher hardness although
less impact resistance than the above.
Caution! These overlays may be difficult or impossible to machine.
Among various compositions offered on the market, EFe2 and EFe3
have been used successfully. AWS ER420 is quite popular,
being also mildly corrosion resistant.

White cast irons are used for metal-to-earth abrasion resistance.
Main ingredients are Chromium, that can range between 6 and 35%,
and Carbon usually between 2 and 6%.
The low carbon alloys are preferred for moderate abrasion
and impact. Higher carbon white irons are selected where impact
is not an issue but where severe abrasion takes place.
Additional elements that may be found are Silicon, Molybdenum
and Manganese. Some of these alloys are specified as ERFeCr-A3,
ERFeCr-A4(Mod) and ERFeCr-A2.

Tungsten Carbides (and recently also carbides of Titanium,
Vanadium, Chromium and other elements) are very hard particles
of selected mesh size, that are embedded by the welding process
into the weld pool of the matrix material. For best endurance,
the carbide size should be smaller than the abrading particles
size. It is important not to melt them in order to preserve
their exceptional hardness.

Therefore the oxyacetylene process may be preferred when applicable,
as is a process involving pouring carbide powder through a funnel
directly into the melt, skipping the passage through the arc.
Tungsten carbides are important for sliding and earthmoving
applications like plowshares and for rock crushing drill bits
and in general for applications requiring maximum abrasion
resistance under low or moderate impact.

Besides the type of the carbides, their size and the welding
process employed, also the volume fraction in the overlay is
important for performance and for comparison in any given
application. As heavy carbides tend to sink, it may be important
to produce shallow melts.

Of the nonferrous alloys, cobalt base hardfacing alloys are most
versatile, resisting heat, maintaining hardness up to 815 0C
(1500 0F) and oxidation up to 1090 0C
(2000 0F), thermal shock, abrasion, erosion, galling,
impact and wear. These carbide containing alloys depend on the
amount of Carbon present for the volume fraction of carbides
of different types and compositions: they present high hardness
at room temperature and various degrees of abrasion resistance.

Another type of cobalt alloys employs Laves Phases, which are
hard intermetallic compounds of exceptional abrasion resistance:
unfortunately it is quite difficult to deposit sound overlays
without cracking except for small applications with correct
preheating so that impact resistance and ductility result quite
low. Therefore this type of hardfacing is preferentially deposited
by metal spray.

Cobalt alloys should never be used for hardfacing titanium
base metal because of the production of brittle intermetallics.

Nickel base hardfacing alloys contain iron, chromium, boron
and carbon. The hard phases present are borides and carbides
that exhibit excellent low stress resistance to abrasion,
generally increasing with their volume fraction. Their resistance
to galling in metal to metal wear is moderate and their corrosion
resistance is lower than that of cobalt base alloys. Some of the
nickel alloys are known by the designation ERNiCr-C, ERNiCr-B and
Alloy 40. Carbide containing hardfacing Nickel alloys found
applications in nuclear power industry as a substitute for Cobalt
alloys because these are prone to become radioactive and to
interfere with normal operation.

Of the copper base two aluminum bronzes are used as hardfacing
alloys on gears, cams and special dies. Two of these overlay
alloys are known as ECuAl-B and ECuAl-D.

Check list for describing the problem.

A number of questions must be addressed when dealing with a
problem of hardfacing, either if trying to solve it by research
through available information or for submitting to a consultant
or to a manufacturer to obtain their recommendations.

It is recommended to try to characterize as much as possible
the problem at hand, by filling in most of the relevant items
of this check list:

Description :
- of the part to be hardfaced
- of its base material
- of original hardfacing if any
- of the most important operating conditions.

Total cost analysis should be performed for each one of
the proposed products and then listed in order of increasing
total cost. An estimate and possibly a test should preferably
be performed to evaluate the practical performance of all products.

The items to include in the cost analysis are as follows:

Definitions:
- Deposition efficiency (varies from 60-70%for SMAW, to 85-90%
for FCAW, to 90-95% for GTAW GMAW and SAW).
- Operation efficiency in % is the actual time of welding
divided by total time including all job activities and
idle time (may vary between 20 to 70% depending on process,
mechanization, work flow etc.).
- Deposition rate (in unit weight per hour):
(weight of consumed material per hour x deposition efficiency = hardfacing material
actually deposited per hour).

Following the Short Item on Passivation
presented in the last issue of PWL,
you may get a deeper look at the subject by reading
"How to passivate Stainless Steel Parts".
Just copy and paste the following
http://www.mmsonline.com/articles/100304.html
on your browser and click Enter.

Repair welding of complex structures is certainly
a major undertaking. See important remarks on
"Optimizing Repair Welding in oil Refineries"
by copying the following address to your browser
http://www.thefabricator.com/xp/Fabricator/Articles/Welding/Weld03/03web131.xml

As announced in a Short Item presented in this issue,
a study describing Weldability and Corrosion
Research on welding 316L can be found (in pdf. format)
by clicking here.

If you are interested in Failure Analysis and would
like to see a full single Issue of a Journal dedicated
to this topic, and downoad it your computer, then follow
these steps:
Open the ASM Home page at
http://www.asminternational.org/
- Click on Technical Journals in the left column,
- Click on E-Journals in the left column,
- Click on Practical Failure Analysis,
- Click on Volume 2 Number 6 December 2002 Free
- Download what you like.

One reader kindly sent the following link to
an Article concerning a litigation related to
welding. See it at
http://www.mrotoday.com/mrowired_advertiser.htm
by clicking on the link to: "Welders file suite to..."

All manufacturing facilities, independent of activity and size,
should implement an established procedure for accepting materials.
It is understood that this Quality related operation, although
seemingly involving an outlay, may actually save untold expenses,
anguish and time.

An anedoctal report was released to the press a few years ago,
when it was found that the main mirror for the Hubble Space
Telescope did not undergo the thorough acceptance test it should
have, probably because of time or budget pressure.

This particular oversight was quite expensive, because, as soon
at it was found that the mirror did not meet specification
requirements, it was necessary to implement a costly repair
procedure involving the addition of not anticipated lenses,
and a special Space Mission had to be scheduled to have
astronauts perform the repair.

But even the simplest of the shops receiving the most common goods
can be thrown into unnecessary turmoil by the mistaken delivery
of substandard or unintended material.

Although the strictest procedures were developed in the aircraft
and space industry, where any minor oversight might have dire
consequences, endangering people and missions, a minimum of
control should be always applied because, in this imperfect
world, mistakes do happen.

It all starts with the material purchase order. The problem
is not with what you write in the order: the problem is what
you DO NOT WRITE. Better writing a loose and very
comprehensive Specification than no Specification at all.
If it is not included there, then for the supplier anything goes.

Especially in these times of global marketing, anyone may have
the temptation to look for less expensive materials. Nothing
wrong with that, as long as it is you who make the decision,
hopefully after satisfying yourself that your product is still
as good and useful as it used to be.

But suppose it is your supplier who gets a stock who-knows-wherefrom,
and it may be even cheaper, and you may get a discount:
how will you know that something is different? Only if you
have the practice of an established Receiving Inspection procedure.

You may ask for and obtain an original manufacturer's
Certificate. As long as everything is OK you will just file
those papers: but if you ever see something strange, or different,
then you are going to ask a few questions. And you may decide
to run a simple test. Is the hardness OK? Does it bend without
cracking? Is the surface clean as it should be?

It is easy to check the Heat Number: it should be stamped
on the bar ends or roll-stamped on the sheets. Examine
dimensions, tolerances, even weight.
A difference in weight may point to a mix up of materials.
Sometimes you may find color codes, or you may establish
them for your facility, if the risk is real to take a
material for another.

Sometimes packing is most important, as for low hydrogen
electrodes that should be kept dry until they are used.
Or for rolls of welding wire, where cleanliness is essential.
Is the description OK? Is it labeled correctly? Are there any
suffixes of which you do not know the meaning? (Ask the supplier).
If it is welding consumables, a functionality test is in order:
Does it weld and flow as you are used to? Does it need
different parameters? Does it produce cracks or porosity?

It should be known that not all consumables are manufactured
equal. This applies, for one thing, to Mig wires (for GMAW):
it may be the consistency of chemical composition,
it may be the standards of cleanliness, it may be the
"throw and cast" (characteristics of wire spooling) that may
influence how the wire is fed by the feeder. It is recommended
that, even if you are satisfied with what you receive from a
certain manufacturer, you test every new batch to be sure not
to have surprises.

And before moving to a different supplier you should make
some practical tests to demonstrate that you will not get caught
unprepared once deep into production.

Some materials, like paints, rubber and other items, have
to be kept at cool temperature, possibly in a refrigerator.
This is also the case of brazing pastes with binders.
Other goods have expiration dates, set by Standards at
some agreed time after manufacturing: how will you know
if the expiring date already passed when you receive the goods?

From time to time you may need to actually test the
composition of a material. While a full chemical analysis
provides the answer, to be checked against the specification,
there is a less expensive qualitative testing procedure,
non destructive, called X-Ray Fluorescence, that can sort
out alloys different from what required.

The equipment may be expensive, but the service is usually
inexpensive, and it is available from a lot of sources,
mainly metal service centers, foundries and even junk yards,
besides metallurgical laboratories. If you wish to refresh
the information given in our Site, you can click on Material Identification.

It should be understood, however, that this identification
cannot take the place of a full certification from the material
supplier, and that it may be dangerous to reissue for
manufacture material put aside long ago and now without
traceability.

If the material responds to heat treatment, an easy test
consists in performing a routine reference heat treatment
and afterwards testing hardness: if different from what
it should be, as long as the process parameters were
correct, then most probably something is wrong with the material.

Practically there is some paperwork involved. Every purchased
item being used in Production should be identified in a
central ledger or catalogue. And then any new delivery
of each one of the items must be recorded with all the
pertinent data, to make exceptions easier to sort out
and to deal with.

A good Receiving Inspection procedure, continued and
consistent, is not an expense, it is a valued asset
considered an essential element of any Quality Assurance program.

Our Welding Advisers Site is never static: it grows with
the addition of items and of pages following questions or
requests addressed to us by interested readers.

This time we announce a new Section on the repair of cracks
in cast iron bodies. We understood it is a hot topic by the
many letters we received asking advice on how to repair a
crack in an old engine block or in a Grand Piano frame, in a
cover for a drain or in an old kitchen range.

Now this Section is included in the page on Welding Cast Iron,
available by clicking here.

A brand new page on those difficult to weld Alloy Steels you
can now reach by clicking on Alloy
Steel Welding

We took the time to include in the existing pages of the Site
the exact links to important articles published in previous
issues of PWL, and so we plan to continue to do in the future.
In this way we enlarge the scope of the information available
to our readers, making additional knowledge easily reached.

All pages links, including those concerning how this Site was
built, how to make a Site that sells, and what tools were used, are
best reached from the Site Map. Click on Site Map.

Let us know on which subjects you would like to see a dedicated
page in the Site. Use the Feedback Form 1. Click here.

Continuing as in the last issue, we will deal briefly
with different subjects that were raised by readers
as feedback on items of interest.

9.1 - Electrodes for GTAW (Tig) are made by Tungsten
either pure or with addition of oxides of Cerium, Lanthanum,
Zirconium or Thorium. AWS A5.12 specifies types, diameters
and current (for argon). Electrodes should be used near their
maximum current carrying capacity. If the current used
is too low, the arc will wander on the electrode tip
surface, producing arc instability. Cleanliness and
smooth finish are most important.

Effect of polarity: at any given current the electrode
connected to the positive side (reverse polarity) is
hotter than the same connected to the negative side
(straight polarity): because of this, at any given
current level, size is one higher when positive,
than that used for negative connection.
Although open circuit high voltage will minimize work
contamination by electrode when touch starting the arc,
it is better practice to use high frequency with no
contact at all.

Oxidized tungsten electrodes should never be used without
cleaning them because they tend to contaminate the weld
puddle. However, the oxides of special elements added
on purpose into their composition (Thoria, Zirconia etc.)
migrating to the surface enhance electron emission and
reduce electrode temperature, improving arc starting and stability.

The most common electrode is probably the 2% thoriated
tungsten. However being thoria mildly radioactive, there
has been a drive to develop other non radioactive materials.
Pure and zirconiated tungsten electrodes are mostly used
with AC. All the other types are used for DC.
Although many different parameters can be studied on
electrodes manufactured by different sources, it is
only by testing that a welder can establish which type
and size perform best for a given job: the elements to be
evaluated in testing are generally the ease of arc starting,
the quality of the weld beads obtained and the useful life
of the electrode at a given current level.

9.2 - Orbital tube welding (OTW), is a mechanized system
that permits to weld stationary tubes end to end or to tube
sheets or to fittings. It consists generally in Weld Heads
suitable for a certain range of tube diameters
containing a special GTAW (Tig) torch which orbits around
the joint. A weld schedule has to be established to ramp
the current up from zero to the maximum appropriate for
the thickness, and then, after some more than a full turn,
to ramp it down back to zero. Depending on the requirements,
special Heads can be used that allow multiple pass welding,
as could be done manually. Welding is generally performed
autogenously (without filler metal) but when needed a wire
feeder can be added. For pressure resistant piping and for
heavy joints, orbital welding is being done with special
GMAW (Mig) or FCAW units, at much higher deposition rates.
Power sources of different types can be employed, with the
coordination being assured either manually or by microprocessor
control.

9.3 - Heat Affected Zone Cracking is concentrated in the
volume of base material immediately adjacent to the weld.
Although the metal composition in the HAZ is not altered
by the weld, the behavior of the Heat Affected Zone depends
on the thermal cycles produced by the weld process as modified,
if applicable, by additional heat input in the form of preheat
or postheat. Furthermore, if hydrogen gas is evolved during welding,
it can easily migrate to the hot HAZ and contribute to cracking.
Therefore it is imperative to use only low hydrogen
consumables, maintained in dry conditions or dried per
manufacturer's instructions before use. In steels the
susceptibility to cracking depends also upon the Carbon
and alloy content of the base metal, usually expressed
in the form of Carbon Equivalent (CE), on the thickness
of the joint, on the level of mechanical restraint and on the
heat input. Preheating can be applied to minimize cracking.

9.4 - Preheating is a technique involving the addition of
heat before welding, usually at a limited temperature,
for obtaining definite advantages when welding structures
of hardenable steels or cast irons that can develop cracks
if special precautions are not applied. Different materials
and processes need suitable preheating procedures.
The main functions of preheating are as follows:

to provide thermal expansion to the volume surrounding
the joint in order to reduce the temperature difference
between the structure and the weld bead and therefore
the shrinkage stresses developing when the melt solidifies
and contracts.

to reduce the cooling rate of the weld and of the hot
material surrounding it, in order to avoid or modify the
hard phases (like martensite) that form upon quenching in
high carbon and alloy steels or cast irons.

to reduce the hardness of any hard constituent by
tempering the heat affected zone at a temperature capable
of softening the microstructure.

to permit to hydrogen to diffuse outside the joint.

9.5 - Tests for tubular Welding are of the most different
types. The tests required to qualify welds employed for a
particular structure depend upon the function, the stresses
and applicable Standards and Codes. It is understood that
the design of tubular elements and joints has to meet
requirements of the Authority supervising
construction. Here we shall review only the most common of the tests
performed on welded joints, excluding those to be performed by
welded tube manufacturers.
The welder should always satisfy him/her/self that full
penetration is achieved, as generally required. This is most
easily done by cutting (with an abrasive disk) sections of
test pieces to reveal the root of the weld: production
welding should not be attempted until the tests are less
than fully satisfactory. Other destructive tests for
prototypes include crushing, tearing apart, opening up by
force to reveal internal defects in the weld beads.
On real production visual inspection is most important
to assess completeness, uniformity, absence of visual defects
(undercut, misalignment, incomplete weld etc.).
Then, according to the function and importance of the job,
non destructive tests may be required like magnetic particles
(for magnetic materials) or penetrant inspection (for
nonmagnetic ones) to detect cracks not visible to the naked eye.
Ultrasonic and Radiographic testing permit to locate defects
at the root of the welds. Hydrostatic or pneumatic pressure
test are used to find leaks. Burst tests are used to find
out the ultimate strength of a welded joint.

9.6 - Ferrite limitations in 316L are generally recommended
to reduce the susceptibility to pitting corrosion in certain
aggressive environments: a generally agreed upon maximum
is as yet not available. However a minimum of delta-ferrite
is usually prescribed to reduce the risks of hot cracking
or microfissuring: this is established at about 3 to 5%.
The solidification modes and the structure types present
in the welds are determined mainly by chemical composition,
as modified by filler metal and dilution. The phases are
examined with reference to the Schaeffler's
diagram, or more recently to a modified constitution diagram
including also nitrogen effect. Other researchers study the
resulting structures based on the ratio between Chromium
equivalent and Nickel equivalent: one must remark however
that calculations of equivalents by empiric formulas may
be based on different assumptions. One such study describing
weldability and corrosion research is listed in the section
"In the press" in this publication.

9.7 - Grade x60 Pipe per API (American Petroleum Institute)
Specification 5L has a minimum Yield Strength of 60 Ksi.
For the root SMA Welding of this grade use only Low Hydrogen
electrodes like E7018 with direct current reverse polarity
(electrode positive).
For fill up passes Flux Cored wire E71T-1, 1.2 mm (0.045")
can be used.
Preheat is always recommended even if not specified.
For thickness 12 mm (1/2") or less, preheat at 38 0C
(100 0F) is required. For thickness over that,
preheat at 93 0C (200 0F).
Minimum interpass temperature should be equal to preheat.
Joint preparation, weld process and welding parameters
should be selected according to tube thickness.

9.8 - Thin sheet welding, to be successful, starts with good
design. Resistance welding is performed on overlapping sheets,
and is most common on cars and consumer items. Two overlapping
sheets or strips may also be fusion welded by melting the edge
of each one to the surface of the other but other solutions
are preferable.
For fusion welding one should select joint configurations that
minimize the possibility of burn through. Butt welding is not
the easiest to weld because it may require a backup bar.
In edge flange welds the sheets edges are presented to the
welding torch when they are parallel and contacting each other:
the sheets are bent as necessary at a distance from the weld
as required. Most of these welds can be performed autogenously
(without filler metal). Corner joints should be avoided, at
least for the thinnest sheet metal. Good fit and adequate tack
welding is most important. Low heat input manual processes
like Oxyacetylene and GTAW are preferred, although mechanized
and automated processes are used for thin formed tubes.

9.9 - Welding procedures are detailed instructions developed
to ensure that welding is performed as required, for obtaining
the designed conditions of performance, stability and durability,
with repeatable and reliable confidence.
Welding Procedures Specifications (WPS) are documents
which must be established by any contractor undertaking to build
structures supervised by regulating Authorities, as demanded by
applicable Codes (like AWS D1.1 or ASME Section IX).
The suitability of those procedures for the intended application
is further verified by practical testing on welded specimens
as required, and the results are presented in Procedure Qualification
Records (PQR) that, when approved, clear the way to
construction. An individual wishing to be accepted for performing
welding on such work has to demonstrate his/her proficiency
in operating the required equipment and in obtaining acceptable
welding results as demonstrated by the performance of needed tests
and as recorded in a valid Welder Performance Qualification
(WPQ) document.

9.10 - Metal Transfer modes for Gas Metal Arc Welding (GMAW)
are characterized by the different ways by which the tip of
the continuous electrode wire is melted by the arc current.
At low current levels metal transfer through the arc occurs
in Short Circuit mode. The advancing wire approaching
the base metal strucks an arc (due to high open circuit voltage) and
a metal drop shorts the circuit estinguishing momentarily the arc.
The molten drop detaches from the electrode opening the
circuit and restarting the arc. It so happens that the drops
form and detach in rapid succession. This mode is suitable
when heat input must be minimal (i.e. for thin gages).
At higher current levels the Globular Transfer mode
takes place. The drop grows to a size larger than the electrode
diameter, and then is dropped without shorting the electric circuit.
Spray Transfer occurs at even higher current
levels: a stream of droplets is directed with force against
the workpiece. This mode gives high penetration, high weld
deposition rate, and high heat input.
Pulse Transfer mode, provided by certain power supplies,
controls low heat input by releasing pulses of high current
(during which metal transfer is of spray type) at a given
pulse repetition rate, while keeping in between the arc at a lower level of
background current, during which metal is not
transferred. This mode is useful for welding thin gages.

Maybe some of you have special experiences interesting and
useful to fellow welders: you are urged to share them with
all of us, as well as comments on what you find in PWL and
what you would like to read.

Readers are invited to inform us of their items of interest
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If you prefer, use Form 1, which is a simple Feedback form, click here,
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We are delighted to see a steady stream of questions
addressed to us from all around the globe.
We would like to provide significant answers but
quite often we are prevented from doing so by
lack of details pertaining to the specific situation.

We have tried to improve on that by providing a Check
List to let the inquirers know what is needed to get a
complete picture. Not many readers use our Form 3.
In a few cases where the questions were so general
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Click here.

It may be that when confronted with details they know
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Would you believe that, after only a few months of
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are very pleased and would like to help everybody.

Please note that we would always like to know if
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But if not yet much helpful, do not give up,
you should persist and ask again, with more details
of what you do and why the answer is not sufficient.

And you are invited to spread the word that here at
Welding Advisers somebody is spending time and efforts
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